Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Porous membrane filtration systems require regular backwashing of the membranes to maintain filtration efficiency and flux while reducing transmembrane pressure (TMP) which rises as the membrane pores become clogged with impurities. Typically, during the backwash cycle the impurities are forced out of the membrane pores by pressurised gas, liquid or both into the feed tank or cell. Impurities may also be removed from the membrane surfaces by scouring with gas bubbles. The liquid containing impurities and deposits from the membranes is then drained or flushed from the tank.
The waste liquid displaced from the tank needs to be disposed of or reprocessed, usually in an environmentally safe manner, so any reduction in the volume of such waste liquid is seen as advantageous in terms of environmental impact and cost.
The draining or flushing of the tank, particularly when large arrays of membranes are used also requires time which results in down time of the filtration cycle. In order to reduce this down time large pumping systems are required to quickly drain and refill the tank. Where tanks or cells are arranged in banks and feed is used to refill the tank, a lowering in levels in other cells may be produced during the refill process. This again impinges on operating efficiency of the filtration system.
Further, in filtration systems employing gas bubble scouring of the membranes it has been found advantageous to confine the bubbles as much as possible in the region of the membranes to assist with the scouring process.
Reduction in backwash volume also reduces the volume of chemical cleaning agents required in some systems. This has the two-fold advantage of reducing cost in terms of chemical requirements while also reducing waste disposal problems.
Minimising the footprint of filtration systems is also desirable in terms of space eventually occupied by the filtration plant. Compact systems have lesser impact on the environment and are more acceptable to the market.
The distribution of gas across large membrane module arrays often leads to complex distribution systems and thus it is desirable to reduce the complexity of such systems where possible.
It has been found advantageous to reduce the volume of feed liquid in the filtration cell as well as confine scouring bubbles as much as possible in order to ameliorate the above problems and provide at least some of the advantages outlined above.